Rigger question...

[John Finch]

I guess you can tell from my last message that it is so top secret that I can't say a word about the extended strut. Hey, on rear sponsons......they are like training whells on a bike. Without them the boat will occassionally flutter. To me, that is the most important need for rear sponsons. I had a great explanation on struts but as usuall my internet connection failed when I sent the message. It seems short messages make it ok, but long ones tend to knock me off line. Anyone know why?

 

[ClayGlover]

Hi John. If you type a long reply and are worried it won't get through, and will be gone forever, this is what I do. Before clicking the dreaded "reply" button, I highlight all of the text I just typed, then right click "COPY" before pressing the reply button. If you're disconnected, you can reconnect, then right-click "PASTE" the message again in the text box on the reply page, or save it in Word for keeping. 2 cents

 

[John Finch]

Hi John.  If you type a long reply and are worried it won't get through, and will be gone forever, this is what I do.  Before clicking the dreaded "reply" button, I highlight all of the text I just typed, then right click "COPY" before pressing the reply button.  If you're disconnected, you can reconnect, then right-click "PASTE" the message again in the text box on the reply page, or save it in Word for keeping.  2 cents

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Thanks,

John

 

[Ray Sametz]

Hey John, Time to break Don's b%lls. His A hole racing that twin was tighter than a 20 needle OOOOOO Ring. Rear sponsons on the day with weather was the ticket. Great race at Atlanta. Ray

 

[Don Templeton]

Hey Gil, I'll ask you or anyone who knows, since I get a reply from Andy Brown about 1 in 3 times if I'm lucky  .  What makes the "anti-walk strut" work as an anti-walk strut?  The extra length?  Thanks

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Good question. It might be magic since the strut isn't in the water.

My boats quit walking when I learned how to set them up correctly.

1- how come there is no off set in the front spondons??? the boats turn to the right so how come no hull off set to the right???

With proper setup there is no need. Also, the weight displacent doesn't seem to be much of an advantage at our scale speeds. Ask this, why would you at this point?
Also, when things are not staright the boat seems to be in a bind going down the straights and then it just exhibits unwanted traits.

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Anti-walk strut? Good question. I will talk about why it does work sometime in the next few days when I've got a little more time.

Preston...It is Magic! hehe

I'll give you something to think about for now. If you run a hydro with the Drive Dog (not the prop) even with the back of the transom and then run the same boat set up with the drive dog 2" behind the transom you will see a difference in the way the boat handles. The effect of the prop is exerted on the bottom of the transom.

Even though the transom is in the "air". And even though the prop is BEHIND the transom in both cases.

Outboard racers could benefit from understanding the effects the prop excerts on the harware in front of it.

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Hmmmm,

I'm beginning to think Andy had second thoughts about discussing this one! :lol: :lol:

 

[Preston_Hall]

We must need to bait the hook.

I'll look at this situation similar to the servo torque thread.

If the prop walk causes the back of the boat to go left then there must be a force acting to the left.

This force acts on the strut end.

The force assumed to be caused by the prop is assumed fixed.

So the further away the prop the more torque the strut sees.

torque = distance (strut length) X force (created by prop)

With this thinking an extended strut should prop walk more.

It's just like when you put a pipe on a wrench. Your force is the same only with increased distance.

Tag! You're it.

 

[ClayGlover]

If you had a 6 inch wrench and a 6 foot wrench, which one would require more force to move to the left of the pivot point? Obviously the 6 foot wrench would be much easier to force left. There must me more to Andy's theory.....

 

[gil sonsino]

In Dynamics we call this Momentum and the effect is exactly the opposite you can't compare it with torque-servo that is static.

Gill

 

[gil sonsino]

<_<

 

[Preston_Hall]

I am not sure we are talking momentum in this case. Sure the boat as a whole has momentum but "prop walk"? I'll assume momentum, but momentum is a vector quantity, mass with direction. This "direction" acts a distance from the strut which creates a moment at the strut axis, i.e. torque.

Actually I thick it is a force acting on the strut. F= mass X acceleration. Prop walk only happens in acceleration.

A car has a rotating wheel, prop in this case, which has a mass rotating at an angular velocity and that results in a tangental force. The tangental force being "prop walk".

This force is only a result of a pressure differential. That is why everything moves......pressure differential. So why does a longer strut have a smaller pressure differential? That may be the question. :huh:

 

[AndyBrown]

Been Purty busy Preston, but I'll get to it.

One thing you mentioned that is partly correct Preston, is that "prop walk only happens in acceleration". The prop is always trying to walk, but it seems most appearent during acceleration. In reality the prop is always trying to "walk" with the same amount of force, but hull and hardware dynamics are constantly changing during acceleration giving the "appearence" of propwalk.

More later.

 

[RIGGERMORTISE]

What about a short strut with loose fitting or worn bearings. The shaft would be able to twist to one side more than a long strut with the same amount of play. It would also be more aparent on aceleration as the stub shaft is forced to one side!

My boats can have a dramatic prop walk problem with the short strut but not as noticabe on the long strut. But then when i tink about it is the prop walk in the same direction as the wear in the bushing? This is making my head hurt :blink:

Just my 2c

Dave

 

[Rickracer]

Ok, I may be taking too simplistic a view of this but let me see if I understand the question.......Let's assume that a surfacing , rotating prop will tend to push the rear of the boat to the left. The moment upon which this force is exerted is the very back of the strut, and anything forward of that is leverage against that force. B)

 

[gil sonsino]

Preston that's correct about the vector quantity and the tangental force so if you keep the whole boat as a constant and move the thrust force(Propellor) further from the transon your result is a smaller tangental force that means less walk i.e,minor propwalk.

The previous input about drive dog location on the transom is the difference when we talk about propwalk.I experienced this on a sport 20 time ago and it's amazing !

Gill

 

[Preston_Hall]

Gil,

I see what you are saying. But isn't this only true if the moment about the strut is constant. i.e. as distance increases the force decreases = constant moment

The way I see it, the prop produces some type of directional force. I would think the force of a prop would be the same no matter the distance from the boat. Therefore, force is fixed, distance increased and thus the greater the moment.

???

 

[Preston_Hall]

Been Purty busy Preston, but I'll get to it.
One thing you mentioned that is partly correct Preston, is that "prop walk only happens in acceleration".  The prop is always trying to walk, but it seems most appearent during acceleration.  In reality the prop is always trying to "walk" with the same amount of force, but hull and hardware dynamics are constantly changing during acceleration giving the "appearence" of propwalk.

More later. 

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Ok, that leads me to what I always thought and that is

1) you need a long rudder, like yours

2) prop position relative to the rudder

3) and I go with Marty on this one. a properly sharpened prop

1 & 3 seem to work for me.

Yes, the prop is always trying to walk. But at a contsant speed there is no acceleration therefore the body is at rest and I didn't want to go there. Mostly because I can't.

You must be busy with Don's 21 again. :lol:

 

[gil sonsino]

so we have many variables that working together contribute for this final balance!

Gill

 

[AndyBrown]

Don's 21 is begining to rise from the dead. It ran 73 mph last night without too much work (72 mph 150 ft out of the turn). His MAC ran 77 in John's boat. Just a little more tweaking and we'll have your old boat cooking Preston. Acceleration was really quick with the new FAT pipe on the boat.

1., 2., and 3. help over come prop walk. #2 really only makes it worse if the rudder/prop relationship is WRONG. However there is only one place where it can be wrong and the other 90% of the locations are OK.

You guys are barking up the wrong tree with the moment arm and leverage ideas.

But keep thinking..... B)

Good Boating,

Andy

 

[Preston_Hall]

Andy,

OK, the stub shaft (bearing surface) is behind the strut mounting center.

Still the wrong tree? :huh:

And now you took Don's boat. He has the ultimate excuse for not running now. :lol: Man, I must have him shak'n pretty good.

 

[topfuel443]

still nothing about the outboards Andy.... ??? or is it in some cryptic code that I havent/cant seem to grasp?

~James